The process of semiconductor manufacturing involves a wide variety of steps including a layer formation process for forming multiple layers such as polycrystalline, oxide, nitride layer, metal, etc., on a wafer as a semiconductor substrate. These steps generally also include a diffusion process, a photolightography process, an etching process, a cleaning process, etc., which are carried out between the steps of layer formation.
Etching is a process in which selected material is removed from a silicon substrate or from thin films on the substrate surface. In one type of selective etching, a mask layer is used to protect specific regions of a substrate on a wafer surface, then a selective etch removes material not covered by the mask. Etching can be performed through two methods, one is dry etching using gas, while another is wet etching using wet chemical. Plasma etching, ion beam etching and reactive ion etching are included in the former, while immersion etching and spray etching are included in the latter.
A common device for wet chemical etching of semiconductor wafers is an immersion chemical cleaning device, also called a wet bench, which includes a plurality of chemical tanks, cleaning tanks, robots, and driers. Batches of wafers are move in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized (DI) water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which high-frequency sound waves are used to dislodge particles from the wafer surface.
One process performed using the wet bench is the “wet etch” process, which involves immersion of the wafer in an organic solution. Usually, a plurality semiconductor wafers are simultaneously immersed into the solution, and the reactants in the solution react with constituent molecules on the wafer surface to produce various gaseous and liquid chemical compounds, which remain in the organic solution after etching is complete. Wet etching of Silicon Nitride (Si3N4) can be performed using a solution of about 85% deionized (DI) water and about 15% phosphoric acid (H3PO4), at a temperature of between about 150° C. to about 185° C.
One problem with the wet etch process is that unwanted silicon nitride or silicon dioxide particulate impurities can be formed in the etchant when the hot phosphoric acid reacts with silicon nitride and when the mask material is being from the wafer. If the particulate matter generated during the etching process are re-deposited on portions of the wafer in which circuitry is being defined, they can cause defects in the circuitry. As device geometries continue to shrink, these particles will become larger compared with the device geometries and the defects will correspondingly be more critical. These defects can result in non-functional electronic devices, which reduce the unit yield per wafer and correspondingly increase the cost of production per unit.